1. Field of the Invention
The present invention generally relates to image scanning apparatuses, image forming apparatuses, and image data processing methods, and particularly relates to an image scanning apparatus, an image forming apparatus, and an image data processing method that remove background noise from image data.
2. Description of the Related Art
Japanese Patent Laid-open Application No. 6-311359 discloses a background noise removing apparatus that removes background noise from image data of an original document after shading correction but prior to predetermined image processing. In this technique, not only the image data below a predetermined threshold is removed as background noise (i.e., the image data above the threshold is removed if the white level is considered higher than the black level), but also γ is gradually changed around the predetermined threshold in an attempt to reduce peculiar appearance of a resulting image at gray-scale levels around the threshold.
Nonetheless, this technique disclosed in Japanese Patent Laid-open Application No. 6-311359 basically discards part of the image data below the predetermined threshold by detecting a background level of the image data obtained after shading correction. Such processing has an advantage of proper reservation of image densities, but γ of the image data undesirably exhibits a large change around the threshold level. Because of this, subsequent image processing is affected, and changes in image levels around the threshold end up creating peculiar appearance.
The reason why peculiar image appearance is created when image data below a threshold level is discarded is that image data ends up having large data value changes around the threshold level, which are further enhanced through subsequent processing of image data that includes MTF (mean transfer function) correction and smoothing. FIG. 1 is a drawing showing an example of changes of image data after MTF correction with respect to a case in which image data is changed below a threshold level and also with respect to a case in which original image data is preserved. When image data shown in FIG. 1-(a) is subjected to the MTF correction, image as shown in FIG. 1-(b) is obtained. When the image data shown in FIG. 1-(a) is changed to image data as shown in FIG. 1-(c) with respect to the threshold level, the image data as shown in FIG. 1-(d) is obtained after the MTF correction, so that the data value changes are further enhanced.
In consideration of this, the technique of Japanese Patent Laid-open Application No. 6-311359 introduces gradual changes to γ around the threshold as previously described, thereby reducing volatile changes of image data around the threshold level and preventing the subsequent image processing such as the MTF correction and smoothing from giving a detrimental effect to the image quality. This measure, however, cannot fully remove such a detrimental effect. Further, since γ is changed around the threshold, imperfection arises in that the image densities cannot be preserved around the threshold level.
In order to remove background noise of an original document, a peak-hold circuit may be used to detect a background level of analog image data, and the detected level may be fed back to an A/D converter as a reference level.
Such a means, however, ends up creating undesirable changes in scanned image levels depending on the brightness of the original document due to an effect of a black offset level and/or an amplifier offset or the like.
Here, a gain of the peak-hold circuit is denoted as A, and values of image data after subtraction of a black offset level is represented as follows with respect to digital data obtained after A/D conversion.“Background Level Output after A/D Conversion”−“Black Level Output after A/D Conversion”=“Background Level+Black Level+Offset Voltage”/“Reference Voltage of A/D Conversion”−“Black Level+Offset Voltage”/“Reference Voltage of A/D Conversion”=((D+Vb+Voff)−(Vb+Voff))/A(D+Vb+Voff)=D/A(D+Vb+Voff)
D: Analog Value of Scanned Background Level of Original Document,
D: Black Offset Level
Voff: Amplifier (or the like) Offset Level
As shown above, the values of digital image data after background noise removal changes depending on the magnitude of the background density D of the original document. Because of this, the extent to which the background noise is removed undesirably varies depending on the background level.